1. Field of the Invention
The present invention relates to a card-type electronic device such as an SD (Secure Digital) memory card, formed of a flat case and a multilayer printed wiring board housed therein, and also to a method for manufacturing the card-type electronic device.
2. Description of the Related Art
A compact SD memory card including a flash memory is known as a medium to store image data, sound data and other data. The SD memory cards are widely used in mobile devices, for example, cellular phones and PDAs (Personal Digital Assistants).
The SD memory card comprises a multilayer printed wiring board on which a memory element and a controller are mounted, and a flat case which houses the multilayer printed wiring board. The case has a recess portion in which the multilayer printed wiring board is fitted. The recess portion has an opening on the outer surface of the case, and a bottom that faces the opening. The multilayer printed wiring board is exposed to the outside of the case through the opening in the recess portion, and it constitutes a part of the surface of the SD memory card. The memory element and controller are housed in a gap between the multilayer printed wiring board and the bottom of the recess portion.
The multilayer printed wiring board has a plurality of conductive layers and a plurality of insulating layers stacked alternately. One of the conductive layers is located at the surface of the multilayer printed wiring board. Since the conductive layer is exposed to the outside of the case through the opening, if the user touches the conductive layer with a finger, the memory element may fatally damaged by static electricity built up in the human body.
To avoid the above damage, in the conventional SD memory card, the conductive layer located at the surface of the multilayer printed wiring board is coated with a solder resist, so that the memory element is protected from static electricity. For example, U.S. Pat. No. 5,784,259 discloses such an SD memory card.
According to the conventional SD memory card, a solder resist must be applied to the surface of the multilayer printed wiring board in order to protect the memory element from the static electricity. Therefore, in the process of manufacturing a multilayer printed wiring board, a step of applying a solder resist is additionally required, so that the number of steps is increased. As a result, the manufacturing cost for the multilayer printed wiring board is increased, resulting in a high-priced SD memory card.
Further, patterned conductive layers slightly protrude from the insulating layers. Therefore, even if a solder resist is applied to the conductive layer, the conductive layer patterns form irregularities on the surface of the SD memory card. Thus, the SD memory card looks unattractive, so that the value of the product is lowered.